Carbaryl, Carbofuran, and Methomyl - National Marine Fisheries ...

addition to the direct impacts that AChE inhibitors have on salmon, there may also be,
independently, significant indirect effects to salmon via their prey (Peterson et al. 2001a). Wild
juvenile salmon feed primarily on invertebrates in the water column and those trapped on the
water’s surface, actively selecting the largest items available (Healey 1991, Quinn 2005).
Salmon are often found to be food limited (Quinn 2005), suggesting that a reduction in prey
number or size due to insecticide exposure may further stress salmon. For example, Davies and
Cook (1993) found that several months following a spray drift event, benthic and drift densities
were still reduced in exposed stream reaches. Consequently, brown trout in the exposed reaches
fed less and grew at a slower rate compared to those in unexposed stream reaches (Davies and
Cook 1993). Although the insecticide in their study was cypermethrin (a pyrethroid), similar
reductions in macroinvertebrate density and recovery times have been found in studies with
AChE inhibitors (Liess and Schulz 1999, Schulz et al. 2002), suggesting indirect effects to
salmon via prey availability may be similar.
One likely biological consequence of reduced swimming, feeding, foraging, and prey availability
is a reduction in food uptake and, subsequently, a reduction in somatic growth of exposed fish.
Juvenile growth is a critical determinant of freshwater and marine survival for Chinook salmon
(Higgs et al. 1995). Reductions in the somatic growth rate of salmon fry and smolts are believed
to result in increased size-dependent mortality (Healey 1982, West and Larkin 1987, Zabel and
Achord 2004). Zabel and Achord (2004) observed size-dependent survival for juvenile salmon
during the freshwater phase of their outmigration. Mortality is also higher among smaller and
slower growing salmon because they are more susceptible to predation during their first winter
(Healey 1982, Holtby et al. 1990, Beamish and Mahnken 2001). These studies suggest that
factors affecting the organism and reducing somatic growth, such as anticholinesterase
insecticide exposure, could result in decreased first-year survival and, thus, reduce population
productivity.
Changes to the size of juvenile salmon from exposure to carbaryl, carbofuran, and methomyl
were linked to salmon population demographics. We used size-dependent survival of juveniles
during a period of their first year of life. We did this by constructing and analyzing general life
history matrix models for coho salmon (Oncorhynchus kisutch), sockeye salmon (O. nerka), and
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